Storage cells of a memory circuit can become defective during and after manufacturing of the memory circuit. Accordingly, conventional memory circuits comprise redundant storage cells to replace defective storage cells. These redundant storage cells are used for fixing defective storage cells directly after manufacturing the memory circuit. In known systems comprising memory circuits coding/error correction is performed to cope with errors occurring later during system operation. The coding/error correction uses redundancy added to the data. Conventional memory circuits are prone to failure when further errors occur and must be exchanged quickly to minimize the probability of a second—then unrecoverable—failure.
More advanced conventional memory circuits provide redundant spare storage cells and use electrical fuse technology to exchange failed storage cells detected by an error detection and correction unit with spare storage cells. These programmable electrical fuses need a lot of silicon area when integrating the memory circuit on a chip. Moreover, it takes a long time to program these fuses during which the memory circuit is not available for operation.
Usually electric fuse self repair uses another small set of redundant storage cells next to the normal redundant storages cells used for repair after manufacturing the memory circuit. This is mainly due to the big size of the electrical fuses with does not allow to place them close to the normal redundancy circuit. Therefore, electric fuse self repair is usually capable of repairing a few bits, but not a complete wordline or bitline. Consequently electric fuse self repair can be used for repair after manufacturing a memory circuit because when soldering an error free memory circuit such as a DRAM on a memory module such as a DIMM the heating of said memory circuit may typically turn few storage cells which had been working before into faulty storage cells repairable by electric fuse repair. However, for long time reliability of the memory circuit, when more errors may pop up during operation of said memory circuit electric fuse repair is not sufficient.
Accordingly, it is an object of the present invention to provide a memory circuit and a method which provides fast repair of a memory circuit when a defective storage cell is detected during operation of the memory circuit.